The invention relates to a process for preparing polar wax products by oxidation of nonpolar polyethylene waxes prepared using metallocene catalysts. For the purposes of the present invention, the term "polyethylene waxes" refers to both ethylene homopolymers and copolymers of ethylene with .alpha.-olefins having a chain length of C.sub.3 -C.sub.18, each having a melt viscosity measured at 140.degree. C. of from 5 to 20000 mPa.s.
It is known that nonpolar polyethylene waxes can be oxidized to form polar waxes. Such oxidation products of waxes and processes for preparing them are described, for example, in U.S. Pat. No. 3,278,513, DE-A-1227654, DE-A-2241057 and DD 283730. The nonpolar starting materials are generally reacted by treating their melts with oxygen or oxygen-containing, possibly additionally ozone-containing, gas mixtures.
As auxiliaries for initiating the oxidation reaction, oxidized polyethylene waxes can be added to the raw material. For example, U.S. Pat. No. 3,692,877 describes the addition of low molecular weight oxidized polyolefins having molar masses of from 500 to 10000. Such oxidized polyolefins comprise long-chain carboxylic acids having average chain lengths of greater than 35.
Depending on the conditions and the duration of the reaction, different degrees of oxidation can be set. The resulting reaction products contain many oxygen-functional groups, e.g. carboxyl, ester, carbonyl and hydroxyl groups. The degree of oxidation is usually characterized by means of the acid number which is a measure of the concentration of carboxyl functions present.
The oxidized polyolefin waxes obtained in this way are employed, inter alia, as auxiliaries for plastics processing or for producing aqueous dispersions, e.g. for use in cleaners and polishers, in textile processing, for waterproofing and for coating citrus fruits.
The polyethylene waxes used as raw material for the oxidation are, for example, obtained by thermal degradation of high molecular weight polyethylene or by free-radical polymerization of ethylene by the high pressure process, also by metal-catalyzed homopolymerization of ethylene or metal-catalyzed copolymerization of ethylene with .alpha.-olefins. Suitable metal catalysts are those of the Ziegler-Natta type or, more recently, also metallocene compounds. The latter contain titanium, zirconium or hafnium atoms as active species and are generally used in combination with cocatalysts, e.g. organoaluminum or boron compounds, preferably aluminoxane compounds. If necessary, the polymerization is carried out in the presence of hydrogen as molar mass regulator.
Corresponding polymerization processes which employ metallocene catalysts are described, for example, in EP-A-321 851, EP-A-321852, EP-A-571882 and EP-A-602509. Compared to Ziegler-Natta systems, metallocene catalysts display extremely high activities. The amounts of catalyst needed are so low that they do not interfere in oxidative further processing of the waxes. Decomposition and removal of the metallocene catalysts, which is associated with considerable expense, can be omitted. The metallocene-catalyzed polymerization allows the synthesis of polymer waxes having widely variable properties, sometimes novel property combinations, so that specific use requirements can be set in a more targeted manner than is possible using conventional polymerization processes. The same is also true for the oxidation products obtainable from such polymer waxes.
A disadvantage which has been found in the oxidation of wax-like polyolefins prepared using metallocenes is the formation of high molecular weight by-products, in the extreme case gel-like, crosslinked by-products. This can lead to an increase in the viscosity of the reaction mixture during the reaction, as a result of which mixing of the reaction mixture with oxygen is hindered and the reaction rate is reduced. Furthermore, deposits are formed on the walls and internal fittings of the oxidation reactor and the use quality of the products, for example the color, are impaired. This behavior is observed particularly when the reaction is carried out in an economically advantageous manner using air as oxidant and at atmospheric pressure or slight superatmospheric pressure.
It has now surprisingly been found that the disadvantages indicated can be avoided by adding a low concentration of inorganic or organic acids to the reaction mixture before commencement or in the early stage of the oxidation.